1. Field of the Invention
The present invention relates to a hearing aid. More particularly, the invention relates to a single-channel circuit for a hearing aid for persons with an impaired hearing faculty. The circuit of the invention has a pre-amplifier and an output amplifier, a microphone and an earphone, as well as a rectifier circuit for producing a control voltage for a voltage dependent amplifier or attenuator which is connected to said pre-amplifier.
2. Prior Art
Such circuits result in an amplification which depends on the signal level of the acoustic signals to be amplified, thus avoiding overamplification and keeping the output signal of the hearing aid within a specific volume level. This prevents strong fluctuations in the volume. Additionally, the frequency response in such circuits essentially remains the same over the whole amplification range which impairs the audibility of speech in the presence of louder background noises.
A better adjustment of the frequency response of the amplification to the user's loss in hearing is achieved in hearing aids by providing two or multiple-channel circuits which mostly comprise a volume compression in the bass channel and a linear amplification in the treble channel, whereby both signals are summed before the output amplifier. In these systems the compression is achieved by a voltage-controlled amplifier.
A disadvantage of the known circuits is their costliness. Furthermore a casing housing such a circuit must be sufficiently large which is a considerable disadvantage for hearing aids.